Zeolite-containing catalysts are commonly used in the petroleum refining industry. An important application of such catalysts is their use in hydrocracking processes that convert certain low grade distillates and other feedstocks to valuable transportation fuels, particularly, high octane gasoline, diesel fuel, and premium quality jet fuel. Of special interest with regard to the present invention are those catalysts which use an ultra-stable Y-type zeolite in combination with an inorganic oxide component which serves as a binder for the zeolite. The zeolite component and inorganic oxide component are combined by various methods known to those skilled in the art to form a particulate support to which a hydrogenation component, often a noble metal, is added.
In general, increased liquid selectivity and decreased coke yield is associated with the use of an ultra-stable Y-type zeolite having a high silica content. However, the lower rate of conversion of heavy hydrocarbons associated with these catalysts is undesirable. The reason that the lower conversion of heavy hydrocarbons is undesirable is that in extinction recycle hydrocracking the heavy hydrocarbons accumulate in the recycle feed stream and depress both liquid yield and jet fuel selectivity.
Zeolite-containing catalysts containing an ultra-stable Y-type zeolite, an inorganic oxide binder, and a noble metal along with a method for preparing the catalyst are described in U.S. Pat. No. 4,556,646 issued to Bezman, the contents of which are herein incorporated by reference. The Bezman Patent also reviews other references which generally describe other zeolite-containing catalysts, their methods of preparation, and their use in hydrocracking hydrocarbon feedstocks. Similar zeolite-containing catalysts are also described in U.S. Pat. No. 4,916,096 issued to Hoek et al. The importance of the silica/alumina ratio was indirectly recognized in U.S. Pat. No. 4,738,941 issued to Dufresne et al.
Prior to their use in hydrocracking operations, catalysts as described above are generally subjected to various pretreatments which are intended to increase the efficiency of the catalyst. For example, P. Dufresne, et al. ("SULFICAT and the 4A-CAT Processes: A New Way to Start Up Hydrocrackers" presented at the 1989 ACHE annual meeting) describes a method for pretreating a zeolitic-catalyst using Ni/Mo as the hydrogenation component with a presulfiding step and a passivating step involving ammonia. U.S. Pat. Nos. 3,117,075; 3,213,013; 3,404,085; 3,505,208; 3,778,365; and 3,816,296 also teach the addition of ammonia or ammonia precursors to hydrocrackers to benefit various operational or product quality aspects of the process.